Translucent polycrystalline alumina (PCA) ceramic has made possible present-day high-pressure sodium and ceramic metal halide lamps. PCA when used in ceramic metal halide lamps in place of quartz arc tubes allows for a higher wall temperature of up to 1000-1150° C. which improves the color properties and efficacy of the metal halide lamps. However, because of the reactions of alumina with the rare earth halide fills, the durability and life of such lamps is adversely impacted. Furthermore, the consumption of the lamp fills by the arc tube material can result in a wide distribution of color temperature, color rendering index (CRI), lumen output, ignition voltage rise, etc., within a large number of the same type of lamps.
Ceramic discharge vessels (also generally referred to as arc tubes) for metal halide lamps have shapes that range from cylindrical to substantially spherical. The hemispherical ends of the spherical shape yield a more uniform temperature distribution, resulting in reduced corrosion of the PCA by lamp the fills. Even so, the life of these lamps is less than typical high-pressure sodium lamps. Thus, it would be desirable to find an alternative ceramic material that would provide the benefits of PCA while being less reactive to the metal halide fills.
Aluminum nitride (AlN) has been indicated by several sources as a potential alternative material for the discharge vessel of ceramic metal halide lamps. See, e.g., K. Maekawa, “Recent Progress in Ceramic Materials for Lamp Application,” Proc. of 7th Light Source Conf., 293-302 (1995). Aluminum nitride has very good corrosion resistance and excellent thermal shock properties that would be very useful in high-power lamps. Unfortunately, it has been difficult to produce AlN discharge vessels having the high transmittance required for lighting applications.
U.S. Patent Publication No. 2005/0258759 discloses one successful method that has been used to produce high-transmittance AlN by employing a prolonged post-sintering anneal. Another method described in U.S. Patent Publication No. 2005/0070421 sinters high-transmittance AlN tubes by adding a piece of solid carbon into the sintering boats to produce a very low partial pressure of carbon in the nitrogen sintering atmosphere. However, the very low partial pressure of carbon in the sintering atmosphere is difficult to control with a solid piece of carbon. The carbon piece must be precisely weighed and a semi-static environment created around the AlN part being sintered. Moreover, the vaporization of carbon from the solid piece can lead to inhomogeneous levels of carbon vapor around the AlN part being sintered, e.g., a high dose of carbon vapor may arise in those areas having a direct line of sight to the solid carbon piece and a low dose of carbon vapor may occur in areas which are out of the line of the sight.